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Sep 012016

Although I use graphical on-screen calculators for many calculations, it can sometimes be convenient to perform calculations at the command-line (or in shell scripts). In which case the old tool is expr :-

» expr 3 \* 9 

Very convenient; even though I can do such a calculation in my head there are circumstances where checking with a calculator is suitably cautious. You can of course perform calculations directly in the shell; if you are using a modern shell such as zsh or bash :-

» echo $((3 * 9))

Whilst convenient, such methods do have their disadvantages :-

  • The expr tool takes it’s expression after the shell has had it’s way with interpreting it – which is why I have escaped the “*” to multiply. You cannot put quotes around the expression either as expr assumes it to be a string.
  • These calculations are integer calculations, so you cannot find out what 77/4 is (19.25). Oops! Turns out that if you make one of the numbers in the expression a float, then the result is properly calculated: echo $((77.0/4) -> 19.25.
  • These calculators are limited to relatively small numbers – according to zsh, 2^63 is -9223372036854775808

If you need something a little more sophisticated then qalc (this is the command-line interface for Qalculate!) makes a pretty good command line calculator. It has to be installed with sudo apt-get install qalc and once installed it should be run interactively to get the initial configuration out of the way :-

» qalc
You need the download exchange rates to be able to convert between different currencies.
You can later get current exchange rates with the "exchange rates" command.
Do you want to fetch exchange rates now from the Internet (default yes)? yes
> quit

Once installed you can perform calculations in the same way as expr (although you can enclose an expression in quotes) :-

» qalc "3 * 9"
3 * 9 = 27
» qalc "2 ^ 72"
2^72 = approx. 4.7223665E21
» qalc "0xff"  
255 = 255
» qalc "86400s to hours"                                                                  
86400 * second = 24 h

You can add the “-t” option to prevent qalc telling you the expression it calculated; perhaps more useful in scripts than interactively.


Sep 012016

One of the advantages that ZFS brings, is that it is so easy to create file systems, that you can create them for purposes that you would not previously do. For example, I have an additional file system mounted under my home directory for a certain application that generates a lot of data that I do not need backed up. Because the script I use to back up stuff does not cross file system boundaries (i.e. it does not descend into a directory that contains a mounted file system), I can simply exclude a large amount of frequently changing data by making a file system.

Or I might (as it happens I do not, but I could well do) create file systems for large lumps of data to easily see how much space they occupy – perhaps ~/Pictures. You can run a command like du -sh ~/Pictures, but that is an expensive command (it takes a while) and it tells you how large the files are; not how much space they occupy on disk. And on-disk compression can make that a significant difference! So simply run df -h ~/Pictures if that directory is on a separate file system.

But there is a bit of a gotcha with that. If you create such file systems in the normal way (such as zfs create pool/mikes-pictures; zfs set mountpoint=/home/mike/Pictures pool/mikes-pictures) you risk creating a situation that may prevent your home directory from mounting. If the “child” file system is mounted before the parent, it will not be possible for the parent file system to be mounted when booting.

Instead create the hierarchy properly :-

zfs create pool/h2
mkdir /h2
zfs set mountpoint=/h2 pool/h2
zfs create pool/h2/mike
zfs create pool/h2/mike/Pictures
ls /h2/mike/Pictures

You will also have to fix the permissions, but this is a far safer way of organising things suitable for future file system creation.


Aug 292016

It seems that occasionally GNOME can go a little screwy and its fancy mouse pointer plugin can result in an invisible mouse pointer. Which makes doing anything just a little bit tricky.

If you can open a terminal, enter the command :-

gsettings set org.gnome.settings-daemon.plugins.cursor active false

And all should be well. At least until it decides to turn itself back on again (so make a note of this fix!).


Mar 252016

Recently I have been seeing quite a lot of usage of (to pick out winners of various kinds of competitions; and no I’m not a winner). The documentation on that site are reasonable with regard to pseudo-random number generators but are not quite correct with regard to the source of random numbers under Linux. And for non-cryptographic uses, the following is fine.

The use of momentarily made me wonder how I would do the equivalent at the Unix (or Linux) command-line, and having used the command before, the shuf command came to mind. To be honest shuffling is not what I think of randomisation given how bad I am at shuffling cards, but despite the name, shuf does pretty well at randomising things :-

» seq 1 10 | shuf

The seq command generates a sequence from 1-10 as given. It turns out that shuf can do it itself :-

» shuf -i 1-10

The most common (relatively) use I have for shuf is to pick out a random line or two from a file. By using the -n option, shuf can do this. The following example makes use of an example file which contains a small number of first names :-

» shuf -n 1 first-names 
» shuf -n 1 first-names
» shuf -n 1 first-names

If you have just a small selection to make, you can provide the list on the command line with the -em option :-

» shuf -n 1 -e Male Female

And that is pretty much all there is to it – a simple tool that does just one thing well.


Mar 052016

Just to amuse myself, I’ve been re-reading and re-learning the Unix shell’s ${} detailsand it occurred to me that whilst these were all very well and cute, they very easily lead to impenetrable code. But they are more efficient.

Take the following two ways of getting the current date :-

✓ mike@pica» print -P "%D" 
✓ mike@pica» echo $(date) 
Sat 5 Mar 13:14:38 GMT 2016

It’s not exactly helpful that they return the date/time in different formats. But glossing over that for the moment, which one is clearer? That is right – the second one clearly says that it is going to “echo” the date. Even if this usage is particularly stupid (as date will echo the date all by itself), the second wins as far as clarity goes.

However it is also less efficient – rather than get the date and show it to the terminal, the shell invokes a sub-process to display the date, captures it and then uses it to show to the terminal. In the old days when terminals consisted of printing mechanisms that actually hit a template of a letter against an inked up ribbon against a roll of paper and hoped that the result was readable, this inefficiency could result in very slow code.

But today this level of inefficiency should not make that much difference, and if it does, then why are you writing code in the shell? There are far better languages out there.

In addition, there is a bit of a gotcha with the print -P “%D” option … it only works if you happen to be using zsh :-

✓ mike@pica» print -P "%D"
✓ mike@pica» /bin/sh
$ print -P "%D"
file: option requires an argument -- 'P'
Usage: file [-bcEhikLlNnprsvz0] [--apple] [--mime-encoding] [--mime-type]
            [-e testname] [-F separator] [-f namefile] [-m magicfiles] file ...
       file -C [-m magicfiles]
       file [--help]
Warning: unknown mime-type for "-P" -- using "application/octet-stream"
Error: no such file "-P"
Error: no such file "%D"
✗ mike@pica» /bin/ksh
$ print -P "%D"
✓ mike@pica» /bin/bash
mike@pica:~/.lyx$ print -P "%D"
file: option requires an argument -- 'P'
Usage: file [-bcEhikLlNnprsvz0] [--apple] [--mime-encoding] [--mime-type]
            [-e testname] [-F separator] [-f namefile] [-m magicfiles] file ...
       file -C [-m magicfiles]
       file [--help]
Warning: unknown mime-type for "-P" -- using "application/octet-stream"
Error: no such file "-P"
Error: no such file "%D"
mike@pica:~/.lyx$ exit

Confusing is it not?damascus-unix-prompt

Of course if the shell would intercept common usages such as $(date) and optimise them, that would be perfectly reasonable.

Jan 052016

A bit of a simple one this … if you are looking at converting an Intel hex format file that looks like the following :-


Then it is relatively trivial under Linux (Debian). The relevant tool is probably installed anyway; unless you are not compiling software which may be a marginal activity for weird people but so is converting ihex files. But just in case, you can install it with: sudo apt-get install binutils.

Once installed (or being already present) the conversion process is as simple as :-

» objcopy -I ihex -O binary somefile.hex somefile.bin

Be careful to specify the second file name or objcopy will overwrite the original hex file (don’t ask how I discovered this!).

Dec 102015


You have a a column of numbers that you have produced in some manner such as :-

$ awk '/clean message/ {print $(NF-1)}'

And you want a quick and dirty way of finding the largest number. Well there is a way but it is perhaps the least efficient way to do it, and that is to sort the numbers into numerical order and use “head” to display the first one :-

$ awk '/clean message/ {print $(NF-1)}' | sort -rn | head -1

But frankly there must be a better method. And yes there is if you happen to be using zsh (or possibly others, but this has been tested with zsh). Simply iterate over the values assigning the current value to the “max” variable if the current variable is larger :-

$ max=0; for x in $(awk '/clean message/ {print $(NF-1)}'; [[ $x -gt $max ]] && max=$x; echo $max

You may be wondering why I don’t simply use the ability of awk to perform calculations. Well that is certainly possible, but I may not always be using awk to produce the numbers in the first place, and this is supposed to be a generic recipe.

Nov 142015

I am obviously doing something wrong because computers are not supposed to behave like this, but my Linux containers (despite previous attempts) are booting with IPv6 privacy addresses randomly :-

✓ root@pica» lxc-ls --fancy | grep chagers
chagers   RUNNING  2001:8b0:ca2c:dead::5e11, 2001:8b0:ca2c:dead:f42b:6dff:fe16:2f2d  YES        
✓ root@pica» lxc-stop --name chagers; lxc-start --daemon --name chagers
✓ root@pica» lxc-ls --fancy | grep chagers
chagers   RUNNING  2001:8b0:ca2c:dead:206b:70ff:fe45:7242, 2001:8b0:ca2c:dead::5e11  YES        
✓ root@pica» lxc-stop --name chagers; lxc-start --daemon --name chagers
✓ root@pica» lxc-ls --fancy | grep chagers
chagers   RUNNING  2001:8b0:ca2c:dead::5e11                                         YES        

That is not how computers are supposed to behave!damascus-unix-prompt

Oct 232015

Have you ever wondered if you can tinker with the ps command to change how and what is displayed? No? Well give up reading this post then.

I've known about ps for ages and also the way that the output can be tinkered with, but have not had an excuse to dig into it properly until I was looking for a way for ps to show the Linux container name for each process (don't get excited: ps -o machine is documented but not implemented at the time of writing). 

If you read the manual page for ps you will be quickly distracted by all the different options available. These can be grossly simplified into three different groups of options: which processes to list, what to output, and how to sort the output.

Which Processes?

This can be simplified down to almost nothing; ps on it's own lists just the processes running from the current terminal (window) :-

% ps
  PID TTY          TIME CMD
 2591 pts/17   00:00:00 zsh
13325 pts/17   00:00:00 ps

If you want to display all processes, add the "-e" option :-

% ps -e
  PID TTY          TIME CMD
    1 ?        00:00:03 systemd
    2 ?        00:00:00 kthreadd
    3 ?        00:00:00 ksoftirqd/0
    5 ?        00:00:00 kworker/0:0H
    7 ?        00:00:54 rcu_sched
    8 ?        00:00:00 rcu_bh
    9 ?        00:00:35 rcuos/0
   10 ?        00:00:00 rcuob/0
   11 ?        00:00:00 migration/0
   12 ?        00:00:00 watchdog/0

And lastly (not literally – there are other options), add the "-p" option to list processes by process ID :-

% ps -p 1
  PID TTY          TIME CMD
    1 ?        00:00:03 systemd

Tuning The Output

By default the fields that ps outputs is somewhat peculiar until you realise that the output fields have been frozen in time. The default choice is somewhat minimal; and I'm not in favour of minimalism. And what use is the TTY and the TIME fields?

The TTY field shows you what terminal the process is running on – this was handy on a multi-user system where you could find out who was on what terminal and then write a message directly to their screen. A great way of winding people up, but not so much use these days. And TIME? We're no longer billed for the cpu time we consume, so the time spent running on the cpu is a rather pointless thing to list.

The "-f" option displays more information :-

% ps -f
mike     26486 31092  0 21:24 pts/24   00:00:00 ps -f
mike     31092 31091  0 20:11 pts/24   00:00:00 -zsh

But the output is still somewhat peculiar, and there are other more interesting fields to display.

There are various options for choosing the output format amongst a set of predefined choices, but the best bet is to ignore these and jump straight into selecting the individual fields that you want. These can be found in the manual page in the "STANDARD FORMAT SPECIFIERS" section. Simply list the fields you want after the "-o" option :-

% ps -o pid,comm,pcpu,pmem,nlwp,user,stat,sgi_p,wchan,class,pri,nice,flags
28061 ps               0.0  0.0    1 mike     R+   3 -      TS   19   0 0
31092 zsh              0.0  0.0    1 mike     Ss   * -      TS   19   0 0

Obviously typing this in every time is somewhat less than ideal, but fortunately the authors of ps have already thought of this. By listing the fields within the PS_FORMAT environment variable, there is no need to specify -o :-

% export PS_FORMAT="pid,comm,pcpu,pmem,nlwp,user,stat,sgi_p,wchan,class,pri,nice,flags"
% ps
29440 ps               0.0  0.0    1 mike     R+   5 -      TS   19   0 0
31092 zsh              0.0  0.0    1 mike     Ss   * -      TS   19   0 0

To make this pernament, add this to your shell startup rc file; whilst editing you may as well set PS_PERSONALITY to "linux".

Sorting The Output

According to the ps documentation, by default the output is not sorted. In that case either my kernel's process table is remarkably well organised, or the distributions I use "cheat" and sort the output in process ID order. In the distant past where computers were shared amongst too many people, and the machines themselves were quite slow, it made sense for the output of ps to be unsorted. But it certainly doesn't make sense now.

And the ps command allows processes to be sorted by any field that you can specify in the "STANDARD FORMAT SPECIFIERS" section which conveniently enough you are now intimately acquianted. Simply add the relevant field to the –sort option :-

% ps --sort pcpu
31092 zsh              0.0  0.0    1 mike     Ss   * -      TS   19   0 0
31743 ps               0.0  0.0    1 mike     R+   5 -      TS   19   0 0

With just a short list (and such a low percentage of the cpu in use) it doesn't make sense, but added to -e, it does.

Rather than change the default sort order, I personally prefer to configure aliases to do the job for me :-

% alias pscpu='ps --sort pcpu'
% alias psmem='ps --sort pmem'

Preferring to use an alias here is rather convenient as there doesn't seem to be a way to configure the default sort order – officially there isn't one!

Reading through the ps manual page (during which you will notice many different options referring to old Unix varients) is a reminder of just how long and bitter the fight over which ps varient was the best. And now for a completely irrelevant picture :-


Oct 032015

One thing that has always puzzled me about Linux Containers was why it is necessary to configure the network address in two places – the container configuration, and the operating system configuration. The short answer is that it isn't.

If you configure network addresses statically within the container configuration :-

» grep net /var/lib/lxc/mango/config 
# networking = veth = up = br0 = = =         2001:0db8:ca2c:dead:0000:0000:0000:000a/64 = 2001:0db8:ca2c:dead:0000:0000:0000:0001

Then the configuration within the container's operating system can simply be :-

» cat /var/lib/lxc/mango/rootfs/etc/network/interfaces
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual
iface eth0 inet6 manual

And that works fine.

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